Scientists at the American Geophysical Union conference made it clear on Wednesday that if peak oil isn’t here now, it’s coming very soon. The US reached its peak in 1971, and according to NASA scientist Warren Wiscombe, most estimates place the global oil production peak between 2000 and 2017. While surely problematic for industry, transportation, and agriculture, could peak oil actually be a good thing from a climate perspective? Burning less oil has got to be good for getting CO2 emissions down, right?

Well, that all depends on what we do.

Ken Caldeira of the Carnegie Institution for Science at Stanford says that oil is actually only a second tier concern when it comes to climate change because there’s not enough of it left to sustain CO2 levels at dangerous levels for very long. The real impacts will depend on how we replace oil as it disappears.

“Coal is the big bear on the block,” said Caldeira. “As we approach the end of oil, will we choose coal or will we choose low carbon technologies?”

Coal may be cheap and abundant, as the coal lobby would have us know, but replacing oil with coal-derived fuels would actually increase global CO2 emissions, according to Caldeira. Not only is coal a “dirtier” fuel than oil (coal emits more C02 per unit of energy than oil does), but there are also greenhouse gases emitted in the process of liquification.

Caldeira spoke on Wednesday at the AGU conference about his recent study examining what could happen to the climate if we ran out of oil today. He created two scenarios, one where we replace oil with coal, and one where we replace oil with renewables. Both scenarios assume we continue to use coal for the same purposes that we do today.  Under the oil-to-coal scenario, carbon emissions will actually increase, causing global temperatures to rise three years sooner than predicted under the Intergovernmetal Panel on Climate Change’s A2 scenario, increasing by 3.6 degrees F by 2042 instead of 2045. In his second scenario, where oil is replaced with renewables such as wind, solar, and nuclear, however, the same temperature rise would be delayed 11 years, to 2056.

“Addressing the climate problem means addressing the coal problem,” said Caldeira. “Most future climate change will be the result of burning coal in absence of policy.”